Solar cells can be a viable energy source by utilizing their ability to convert sunlight to electrical energy. Silicon is a semiconductor material and the raw incoming material used in the manufacture of solar cells. The electrical properties of the cells, namely the conversion efficiency depends on the purity of the silicon. Several techniques have been used to purify silicon. The most well known technique is called ‘Siemens process.’ This technique removes substantially all impurities present within the silicon. However, this technique requires production of the silicon in a gas phase and re-depositing into a solid phase in order to remove the impurities. Other techniques include zone refinement, and directional solidification.
Many techniques used for purifying a large quantity of silicon operate on the principle that while silicon crystals are solidifying from a molten silicon solution, undesirable impurities remain in the molten solution. For example, a float zone technique, can be used to make silicon monocrystalline ingots using a moving liquid to urge impurities toward an edge of a mold for removal. Another example technique, the Czochralski technique, can be used to make silicon monocrystalline ingots using a seed crystal that is slowly pulled out of a molten solution, allowing the formation of a monocrystalline column of silicon while leaving impurities in the solution. Further example techniques, such as the Bridgeman or heat exchanger techniques, can be used to make silicon multicrystalline ingots through the creation of a temperature gradient with a controlled cooling rate, causing directional solidification Improvements in purification efficiency and cost are always desired.